Various types of amplifier topologies have been designed to function in a variety of classifications. For example, a Class A amplifier conducts signal current throughout the cycle of the signal waveform (e.g., providing 360° conduction). A Class B amplifier Conducts signal current for exactly one-half of the cycle of the input-signal waveform (e.g., providing 180° conduction). A class AB amplifier generally corresponds to a Class-B amplifier with bias designed to avoid cross-over distortion.
A Class-D amplifier is an electronic amplifier which, in contrast to the active load used in linear mode AB-class amplifiers, uses the switching mode of transistors to regulate power delivery. This feature enables the Class-D amplifier to achieve high power conversion efficiency. For instance, Class D amplifiers use pulse width modulation (PWM), pulse density modulation (sometimes referred to as pulse frequency modulation) or other forms of advanced forms of modulation, such as sigma-delta modulation. A drawback of the Class D amplifier, however, is that very large FET sizes are typically required to achieve high efficiencies and to lower the resistance at turn on.
There are efforts to develop other higher efficiency amplifiers, including Class G and H amplifiers. These amplifier designs seek to combine a class AB output stage with other techniques to realize higher power efficiency with low distortion. The terms “Class G” and “Class H” are used somewhat interchangeably to refer to different designs, varying in definition from one manufacturer or publication to another. Class G amplifiers are a more efficient version of class AB amplifiers by employing “rail switching” to reduce power consumption and raise efficiency. The topology utilizes two or more power rails at different voltage levels and switches between this rails as the signal output approaches each level. Thus an AB amp design can increase in efficiency by reducing the wasted power at the output transistors from a larger than required voltage rail. Class H amplifiers operate by continuously boosting and lowering the supply, as required, above a certain minimum bias level.
Despite the advances made with Class G and H amplifiers, there is still a need for more efficient amplifier designs.